ARM: dts: stm32: Fix display-timings settings for stm32f746-disco

Since commit ef4ce6df3289 "video: stm32: stm32_ltdc: fix data enable polarity"
The panel display output wasn't functional anymore.
Device tree display-timings de-active property value must be updated
to 1.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/arch/arm/dts/stm32f746-disco-u-boot.dtsi b/arch/arm/dts/stm32f746-disco-u-boot.dtsi
index f88466f..a4ce936 100644
--- a/arch/arm/dts/stm32f746-disco-u-boot.dtsi
+++ b/arch/arm/dts/stm32f746-disco-u-boot.dtsi
@@ -58,7 +58,7 @@
 				vsync-len = <10>;
 				hsync-active = <0>;
 				vsync-active = <0>;
-				de-active = <0>;
+				de-active = <1>;
 				pixelclk-active = <1>;
 			};
 		};